Selective synthesis of macromolecules is desirable to tailor their structural and functional properties, such as hydrophobicity, hydrophilicity, and interfacial and film forming properties. E.g., polyesters synthesized by enzymes may be useful as time-release bioerodable and biodegradable drug delivery matrices such as coatings, finishes and films, and biodegradable emulsifiers, compatibilizers and detergents..sup.1 However, selective synthesis of polyesters by chemical reactions are difficult due to multiple steps involved in the modification (protection, deprotection, extraction, separation and purification), and due to the difficulty in controlling molecular weight and polydispersity and hence lack of specificity and solubility of the polymer..sup.2 Moreover, some of the polyesters synthesized by chemical methods are neither biodegradable nor bioerodable.
Enzymes are powerful catalysts in organic solvents where they catalyze a wide variety of reactions that are difficult to perform in aqueous media. This is particularly evident in esterification reactions catalyzed by lipases and proteases wherein a variety of nucleophiles act as substrates for enzyme-catalyzed acyl transfer in nearly anhydrous organic solvents. The development of suitable techniques for the synthesis of polyesters and copolyesters in organic solvents, therefore, would represent both an opportunity for the synthesis of novel materials as well as means to overcome a technical hurdle in the broader uses of enzymes in non-aqueous media.
Polyesters synthesized by enzymes are of great interest for their mechanical properties, biodegradability, bioerodability and applications in drug delivery systems..sup.3 Recent investigations have shown that lactones can be polymerized in presence of enzymes such as lipases..sup.4-7 However, the following serious drawbacks of this technique were reported in the reaction procedures; high concentrations of enzyme in the system; long reaction time required for the polymerization to reach completeness; and one time use of enzyme. Such limitations were prohibitive for an industrial application using the enzymatic path.
Proteases are enzymes capable to catalyze peptide and ester bond synthesis in various organic solvents..sup.8-9 Here we propose a novel approach using low amounts of protease enzyme in an non-aqueous solution to catalyze the polymerization and copolymerization of lactones and lactides in presence of different initiators. In addition, this protease mediated polymerization in solution produces polyesters and copolyesters with defined molecular weight and dispersity. A method is also described for the synthesis of copolyesters, wherein the composition of the copolymer is predetermined by the initial monomer composition of the lactones and lactides. A method to achieve enzyme solubilization in organic solvents through formation of hydrophobic ion-pairs between an enzyme and a charged surfactant was previously used..sup.10 In particular, it has been observed that subtilisin Carlsberg (from Bacillus licheniformis), protease N (Bacillus subtilis), and protease S (Bacillus specific) ion paired with the anionic surfactant dioctylsulfosuccinate, sodium salt (aerosol OT or AOT), remained predominantly active in isooctane..sup.11, 12 In addition, these microbial protease are commercially available and is much cheaper than lipases.
This system, containing proteases ion paired with a detergent, was successfully used for the regioselective acylation of sugars.sup.12 and of polysaccharides with alkyl vinyl esters..sup.13 The ion-pairing in organic solvents of protease enzymes is a valuable tool toward the catalysis of other reactions that implement a polyesterification of a substrate with specific initiators.
Accordingly, it is an object of this invention to overcome the above illustrated inadequacies and problems of enzyme-based polyester synthesis by providing an improved method.
It is the object of this invention to use ion paired protease enzymes in organic solvents for the synthesis of polyesters from different lactones and lactides in reasonable times for industrial applications.
It is another object of this invention to provide a method of synthesizing homopolyesters and copolyesters from lactones and lactides wherein their selective polymerization results in structural and/or functional benefits.
It is object of this invention to provide a method of synthesizing polyesters and copolyesters from lactones and lactides wherein the polymers synthesized are of selective and predictable molecular weight with well defined molecular weight and narrow molecular weight-dispersity and resultant structural and/or functional benefits. Polymers with such well defined molecular weight and dispersity are required for drug and cosmetic preparations or systems for time defined release, breakdown and/or biodegradation both in vivo and in vitro.
It is another object of this invention to provide a method of synthesizing copolyesters from lactones and lactides wherein the copolymers synthesized are of decreased crystallinity and increased processability.
It is another object of the present invention to provide a method of enabling the use of enzymes to catalyze reactions in non-aqueous media for the synthesis of biodegradable, bioerodable and biocompatible polymers.
It is another object of this invention to provide a method of enabling the use of proteases to synthesize polyesters and copolyesters from lactones and lactides, wherein the proteases are commercially available and are much cheaper than lipases.
Yet it is another object of the present invention to provide a method of enabling the use of enzymes to catalyze polymerization reactions in non-aqueous media, wherein the enzyme and the reaction media can be reused and/or recycled for subsequent polymerization reactions.